Change container image USER to UID (open-telemetry#738)

* Change container image USER to UID

Kubernetes requires USER to be UID in order to verify running as non root.
ref https://github.com/kubernetes/kubernetes/blob/master/pkg/kubelet/kuberuntime/security_context_others.go#L49

* Use 65532 as nonroot UID

Dockerfile

@@ -34,6 +34,6 @@ RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 GO111MODULE=on go build -ldflags="-X $
 FROM gcr.io/distroless/static:nonroot
 WORKDIR /
 COPY --from=builder /workspace/manager .
-USER nonroot:nonroot
+USER 65532:65532
 
 ENTRYPOINT ["/manager"]